The complexity of the combustion process has lead to contradictory theories concerning how to best improve combustion efficiency and what role fuel temperature plays in combustion system efficiency, versatility, and robustness.
In several patents issued in the field of combustion devices, a function of the device is to cool liquid fuel. For example, U.S. Pat. No. 5,988,265 to Marthaler is directed to a fuel cooling device and states that “[o]ne reason to lower the fuel temperature is to be able to provide a more uniform and consistent fuel temperature which may be desirable in order to control emissions. Another reason is to be able to provide a fuel of lower temperature for more efficient engine operation at high load conditions.” Similarly, U.S. Pat. No. 6,428,375 to Takayanagi is directed to a fuel cooling apparatus for an outboard motor. However, cooling of fuel to achieve a uniform and consistent fuel temperature is limited in its application. This is because the ambient conditions of the fuel supply dictate the start temperature of the fuel. Thus, for example, the fuel might start out in a cold condition such as outside during the winter. In addition, latent heat from the combustion source often tends to work against, or to counteract, the affect of external cooling devices thereby reducing the chance for the achievement of efficient fuel cooling. In addition, in those cases in which the fuel is to be cooled below surrounding atmospheric conditions, expensive and high power consuming refrigeration type equipment is required. Further, downsizing refrigeration equipment to a point at which it can be installed in the fuel gun of an oil burning heater is not viable. Consequently, there is a clear need in the art for a device that can efficiently provide a more uniform and consistent fuel temperatures since more uniform and consistent fuel temperatures would be beneficial to the combustion process.
On the other hand, in several other patents issued in the field of combustion devices, a function of the device is to heat liquid fuel. Extensive research and development also has been done in the field of fuel vaporizing devices. As a result, in many of the patents issued in the field of fuel vaporizing devices, the function of the device is to add heat to the liquid fuel in such a manner that the fuel is uniformly and consistently heated to its vaporization temperature. Accordingly, the heating of a fuel in such a manner that the fuel is completely vaporized upon entering an airflow has been indicated as being the key factor in achieving maximum combustion efficiency. For example, U.S. Pat. No. 4,396,372 to Matumoto et al. is directed to a burner system that is adapted to vaporize a liquid fuel such as kerosene at temperatures of 250° to 300° C. Similarly, U.S. Pat. No. 4,465,458 to Nishino et al. is directed to an apparatus for burning liquid fuel that is equipped with a fuel vaporizer that is designed to operate in the range of 200° to 250° C. Further, U.S. Pat. No. 4,483,307 to Gilmor and U.S. Pat. No. 4,475,523 to Goranflo are directed to fuel vaporization devices for use in internal combustion engines. The underlying theory in the art related to fuel vaporizing devices is that a fuel in its vapor or gaseous state burns cleanly. In this regard, it has been theorized that maximum combustion efficiency due to pre-heating the fuel is achieved once 100% of the fuel has been vaporized. Nevertheless, it is well known that hydrogen is a gaseous fuel that burns cleanly. Acetylene, on the other hand, also is a gaseous fuel, but it does not burn cleanly in air. Combustion of the gaseous fuel acetylene in air results in the production of solid carbon in the form of a thick black smoke and soot. Accordingly, it will be recognized that there is a need in the art for a device that correctly identifies and optimizes how fuel temperature affects combustion efficiency and overall appliance efficiency.
When one pound of the carbon contained in a hydrocarbon based fuel is burned to completion, it produces 14,500 BTU of heat. Similarly, when one pound of hydrogen is burned to completion, it also produces a specific amount of heat. Accordingly, the state of the fuel is not a determinative factor in the amount of heat released by the fuel in the combustion reaction. For example, 1 pound of carbon in gaseous propane contains the same amount of energy as one pound of carbon in diesel fuel or even bunker oil. The amount of heat released is based on the mass of carbon and hydrogen burned, not the state of the fuel.
Consequently, since the first law of thermodynamics calls for a conservation of energy, it will be understood that an input of a certain amount of energy to heat a fuel can only result in a maximum increase of that certain amount of energy over and above the amount of energy being released by the combustion reaction itself, assuming that the fuel is burned to completion in both cases. Therefore, an increase in combustion efficiency measured in BTUs released per pound of fuel consumed can only be achieved by increasing the percent of fuel burned and not by changing the state of the fuel burned. As a result, if a burner completely combusts 99% of the fuel flowing through it, only a 1% increase in combustion efficiency is possible. Many constant flow pressure atomization type burners on the market today have combustion efficiencies in the 99% range. Nevertheless, there remains a need in the art for a method capable of modulating the flow rates of these burners without sacrificing the combustion efficiencies thereof (i.e., percent of fuel burned).
Droplet size can also affect the percent of a liquid fuel burned in a particular burner. Accordingly, since a droplet of fuel burns from the outside in, if the droplet is exceedingly large and is moving fast enough, it can leave the region of combustion prior to being completely consumed. Reducing fuel pressure to modulate or reduce fuel flow rates, however, increases droplet size and inhibits complete combustion. Hence, heating a liquid fuel to reduce droplet size can aide in achieving complete combustion, but, heating a liquid fuel to the point of vaporization results in a decrease in system efficiency. This is because once the droplet size necessary to cause complete combustion is achieved, the maximum combustion efficiency also is achieved. Accordingly, any heat energy consumed in the course of doing the work of providing any further droplet size reduction is not regained as an increase in the percent of the fuel burned. Therefore, there is a need in the art for a device and method that maintains the droplet size necessary for complete combustion and does not waste additional energy on unnecessary droplet size reduction. There also is a need in the art for a device and method that maintains proper droplet size at lower fuel pressures. Still further, there is a need in the art for a device and method that allow flow rate modulation in existing constant flow burners without sacrificing combustion efficiency.
In the latter regard, it has been found that the introduction of heat to cause vaporization requires unnecessarily high temperatures that in turn result in apparatus functionality problems. For example, tar is produced at the heat/fuel interface when the temperature of the interface is too high. (See, for example U.S. Pat. No. 4,465,458 [Nishino et al.] which claims the use of a special catalyst with a fuel vaporizer to rectify tar production.). However, as alluded to above, elevated temperatures are needed when the heat/fuel interface surface area is too small and/or the target final fuel temperature is too high. Thus, there is a need in the art for a device that solves the tar production problem associated with fuel heating.
It also is known that maximum combustion efficiency is achieved once complete combustion is achieved. However, the efficiency at which the heat generated from combustion is used to do work depends on many factors. For example, in the case of a heater system designed to heat a house, if the heat exchange that transfers the heat from the combustion process to air flowing into the house is only 50% efficient, then there is the theoretical possibility for a 50% increase in the efficiency of the heat transfer or exchange. Current combustion systems typically used to heat homes and for industrial applications are designed to function at a constant flow rate of fuel. This basic fuel control algorithm has not changed since the 1930's. A thermostat senses when the temperature of the room in which it is located has dropped below a lower set point temperature. When that occurs, the burner ignites and runs at a constant fuel flow rate in all conditions until the temperature of the room in which the thermostat is located reaches its upper set point temperature. Changes in the flow rate of the fuel in the foregoing situation can have a positive effect on the heat transfer and efficiency of the heater system by increasing run times and decreasing the number of light-offs. In addition, increased run times can increase the comfort of heated environments by reducing room temperature fluctuations. Hence, there is a need in the art for a device that can be adapted to existing constant flow burners so as to convert them into modulating burners whereby the heat output of the system can be modulated to react to variations in the demand for heat. For example, variations in the demand for heat can be made dependent upon changes in weather conditions (for example, very cold days require more heat than mildly cold days) or in the most efficient fuel flow control algorithms. (for example, it is often more efficient to have a high initial fuel flow rate to quickly reach thermal stabilization which is then modulated to a lower fuel flow rate to increase burn time.
Conversely, however, in a hot water heater context, it may be more efficient to have a low initial firing rate to react to a low water flow rate such as that encountered when a person washes his hands that switches to a higher flow rate if the lower firing rate is not sufficient). Additionally, there is a need for newly developed burners that are designed for modulated fuel flow rate operation.
New devices are being developed that use fuel pressure changes to modulate fuel flow rates. Exemplary devices of this type utilize pressure ranges from about 100 psi to about 600 psi. In these devices, the burner operates, for example, at a fuel pressure of 600 psi to achieve a flow rate of 0.75 gph. In such a case, the pressure must drop to 100 psi in order to achieve a flow rate of 0.4 gph. These very high-pressure systems have been found to have many inherent problems. For example, if the pressure is elevated, then the fuel exit orifice must be very small in order to maintain the desired small fuel flow rate. However, tiny fuel exit orifices tend to clog. Further, there is no way to predict when the orifice is becoming clogged, so no-heat situations resulting from clogged fuel exit orifices are unavoidable. Additionally, fuel droplet size changes dramatically when the pressure changes from 600 to 100 psi. Finally, high fuel pressures require stronger fuel pumps and better fuel connection sealing techniques to eliminate fuel leaks. The resultant no-heat situations and after-hours service calls resulting from these system weaknesses are a significant and continuing problem for both the customer and the provider in the oil heat industry. Hence, there is a need in the art for a device and method that remotely monitors burner function and predicts system malfunctions thereby allowing for the scheduling of preventative maintenance, rather than unscheduled after-hours service calls due to system failures. Consequently, there is a need in the art for a device and method that can modulate fuel flow rates without resorting to high fuel pressures.
When modulating fuel flow in a burner, it is also necessary to modulate air flow rates to maintain the proper air/fuel mixture ratio so as to achieve efficient combustion. A significant amount of research has been done in the field of air flow and how air flow must be changed when fuel flow is changed in a burner. For example, U.S. Pat. No. 4,464,108 to Korenyi discusses a flame retention head that employs two sets of swirl vanes to increase mixing. Also, U.S. Pat. No. 4,484,887 to Patterson employed shields to affect the air stream. Further, U.S. Pat. No. 6,382,959 to Turk discusses a burner airflow adjustment. Still further, U.S. Pat. No. 5,184,949 to O'Brian discusses airflow adjustment. However, a reduction in fuel flow requires a reduction in airflow such that the proper air fuel mixture ratio is maintained. In some cases, this airflow requirement is below the mechanical limitations of the burner. Further, traditional flame retention head designs rely on a pressure drop across the head to cause the vortex flame shape and proper air fuel mixing, but the low airflows required at fuel flows less than 0.5 gph often result in diminished effectiveness of the flame retention head. Consequently, there is also a need in the art for a device that allows for airflows below the mechanical limitations of traditional burners.